Why is it that there apparently are some poskim (Halachic authorities) that hold Noled (something that didn't exist before the onset of Shabbos or Yom Tov) is Muter on Shabbos but Ossur on Yom Tov? (At the moment I don't have any good sources to back up this claim...)

2 Answers
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The Gemora (Beitza 2b) explains that since Yom Tov in general is more lax than Shabbos, they were more stringent about mutkzah in order that people should not disregard it. See Shulchan Aruch Harav (OC 495:13) for the various opinions l'halacha.

Note that it's not just nolad that's more machmir. There is a machlokes generally by muktzeh between R' Shimon (lenient) and R' Yehudah (machmir). On Shabbos we follow R' Shimon, and on Yom Tov we follow R' Yehudah.

The gist of the machlokes is whether your intentions affect the muktzah status of an item. For example, if you have a pile of seforim but you have them to sell, not to read, R' Yehudah says that is muktzeh since you didn't have intention to use them on Shabbos/Yom tov, and R' Shimon says it isn't because he doesn't hold of that restriction.

So lemaaseh, on Yom Tov when we go like R' Yehudah, whatever could be classified as not intended to be used when Yom Tov came in, will be muktzeh. Obviously ask your LOR for specific cases.

(I don't have the sources here, I think it's the second or third siman in hilchos yom tov in Tur/Shulchan Aruch - see there)